Spherical Surface Measurement Based On Pinhole Diffraction Interferometric System

With the development of national advanced scientific projects,such as aerospace,ultraviolet lithography and laser nuclear fusion,the demand for high-precision spherical optical elements is increasing rapidly.High machining precision is put forward,and higher measurement accuracy is needed.The point diffraction interferometry(PDI),which use a nearly ideal spherical wavefront diffracted by a micro-size pinhole as the reference wavefront,theoretically can achieve nanometer or sub-nanometer surface measurement accuracy.It provides an excellent way for the superprecision measurement.At present,the research of PDI is still in the laboratory stage,how to obtain a ultra-high precision diffraction reference wavefront and realize the high precision processing of the interferograms are two important aspects in the research of PDI.This research is focused on the 3D analysis of point diffraction light intensity and diffraction wavefront,the construction of spherical point diffraction experiment system and the high-precision processing of point diffraction interferograms.It contributes to the development of practical point diffraction interferometer.Firstly,based on Rayleigh-Sommerfeld vector diffraction theory,the analytical models of the diffraction intensity and wavefront error of the pinhole with three-dimensional alignment error are established.The influence of different diameter pinholes on the distribution of diffraction intensity and wavefront error is discussed.The light intensity distribution and wavefront error distribution of pinhole diffraction stemming from three kinds of misalignment errors including shift,defocus and tilt are analyzed.The study provides an important theoretical basis for the construction of the point diffraction interferometry system.Secondly,a point diffraction interference measurement system was proposed for the highly reflective spherical mirror surface.The experimental system is established.It is adjusted accurately and the phase-shifting interference fringes were obtained.The algorithm of point diffraction phase-shifting interferograms processing and the realization of self-programming are studied.The auto-extraction algorithm of circle domain and the phase extraction algorithm of non-synchronization number are discussed.The region growing algorithm is compared with the least square phase unwrapping algorithm based on DCT.The fitting effect of 2?6 order Zernike polynomials on spherical and aspherical surface deviation is simulated and compared.Finally,the point diffraction interferograms are processed by the circle domain extraction,7-step phase extraction,region growing unpacking and 36 items Zernike polynomial fitting algorithms.This is compared with IntelliWave software,and the surface shape of the two is basically similar.The difference of PV value is 0.0026? and RMS value is 0.0058?.It is verified that the accuracy of the algorithm can meet the high accuracy requirement of the image processing algorithm for point diffraction interferometry measurement.In addition,a comparative experiment is carried out on the ZYGO interferometer.The experimental results are very close to the experimental system in this paper.The mean error of PV is 0.004?,and the RMS is 0.004?.